When prohibited food must be eaten because of life threatening circumstances, the rule is "ma'achilin lo ha'kal ha'kal techila" (Yoma 83) – if there is a choice of issurim, we choose the lesser evil. This principle gives rise to a number of disputes in the gemara and rishonim as to which among various choices is the more minor issur. One of the interesting cases involves a person who requires meat on Shabbos, but there is no kosher meat available -- would it be better to violate Shabbos and shecht an animal to have kosher meat, or to consume non-kosher meat without violating Shabbos? At first glance one would think that Shabbos, an issur sekilah, is certainly the more stringent issur, but the Ran disagrees and argues that eating the treif food is more stringent as each and every k’zayis is a separate issur, while the Shabbos violation would involve just one act. This seems to be a classic question of kamus vs. eichus: how does one qualitatively more stringent issur stack up against a greater quantity of lesser issurim?
The Parashas Derachim challenges the Ran based the following case: The gemara asks which is worse -- eating tevel or eating terumah? Says the gemara: if it is possible to fix the tevel and make it chulin, there is nothing to talk about, as that is obviously the best option. The case in question is where that is impossible. Is tevel the lesser issur because at least in potential it can be transformed into chulin, or is terumah the lesser issur because at least it is permitted for some people (kohanim) to eat b’heter? The gemara quotes a machlokes and does not resolve the question.
What type of case does the gemara have in mind when it says that the tevel cannot be corrected through hafrasha? Rashi explains that we are speaking of a case where someone is ill and requires a specific amount of food which is equal to the total quantity of tevel available. For example, imagine a person who must eat an entire challah roll, and all you have is enough flour to bake exactly that one single challah roll. If you separate off terumah from the flour, the person will still have to eat that portion as well. Is it better to just bake the roll as-is, in a state of tevel, or is it better to do a hafrasha of terumah even though the person will have to eat both the chulin portion and the terumah portion?
Returning to the Ran, if multiple quantities of a lesser issur are viewed as more severe than even a one time desecration of Shabbos, it stands to reason that eating the one bite of terumah would be preferable to taking multiple bites of tevel, each of which is a separate issur. What's the whole machlokes about? Shouldn’t everyone agree the one bite aveira of terumah is the lesser evil than the bite after bite multiple aveiros done by eating tevel?
The Parahas Derachim answers that the Ran did not learn the gemara's case like Rashi. The machlokes must be where you have two separate portions, one which is terumah, one which is tevel, and are not sure which one to use. Why then not just separate terumah from the tevel? What makes it impossible? The P.D. says that there must be some technical reason that makes hafrasha impossible, e.g. the owner of the wheat did not give permission to do hafrasha from it, in which case the hafrasha is not chal. According to the Ran, given that there are two piles, no matter which one you eat, there will be multiple issurim involved.
Now we get to the fun part. The Parashas Derachim adds that by learning the gemara in this way (i.e. not like Rashi), you can resolve another difficulty. Given the way Rashi sets up the case, it seems that it is in fact possible to have your cake and eat it to without violating any issur!Instead of being mafrish terumah and eating it separately, why not just mix the terumah back with the other flour and be mevateil it?! True, there is a din of ain mevatlin issur l’chatchila – you cannot intentionally set up a situation of bitul – but that is only a din derabbanan according to most Rishonim. Everyone should agree that bitul is a better option than a certain d’oraysa violation. Why according to Rashi is this not a valid solution? It must be that since the gemara does not consider this option it means that we are not speaking of a single pile of food, but rather of two distinct piles, one of terumah, one of tevel, and the question is which one to choose.
This issue of ain mevatlin issur l'chatchila on terumah gets us involved in a major debate among Achronim -- to be continued bli neder.